1. Field of the Invention
The present invention relates to a near infrared screening filter glass which is useful, for example, as a color-correcting filter glass for a color VTR camera. More particularly, it relates to a near infrared screening filter glass which is capable of efficiently transmitting visible rays of from 400 to 520 nm and has excellent properties for absorbing light rays of about 700 nm and which has good weathering properties.
2. Discussion of Background
The spectral sensitivity of a picture-taking device used in a color VTR camera usually extends to cover from the visible region to the near infrared region (1000 nm). It is, therefore, necessary to screen light rays in this near infrared region by means of a filter to bring the sensitivity close to the visual sensitivity of human being. For this purpose, filter glass having CuO incorporated to a phosphate glass, is used for a filter of this type so that near infrared rays are selectively absorbed. Specifically such a filter glass is required to have a high transmittance of visible rays of from 400 to 520 nm and to be able to absorb light rays of from 550 to 950 nm as much as possible.
However, conventional near infrared screening filter glass is a glass containing a substantial amount of P.sub.2 O.sub.5 and thus has poor weathering properties. It thus had a problem that, for example, the surface underwent a modification when left at 65.degree. C. in an atmosphere having a relative humidity of 90% for about 250 hours, and as the time further passed, the spectral properties tended to deteriorate. Whereas, a fluorophosphate glass containing CuO has been proposed as a near infrared screening filter glass which satisfies both of the spectral properties and the weathering properties (Japanese Unexamined Patent Publication No. 219037/1989). This glass does not undergo any surface modification or deterioration of the spectral properties when maintained at 65.degree. C. under a relative humidity of 90% for about 1000 hours. However, this glass has a drawback that when subjected to a powder water-durability test in accordance with JOGIS (Japan Optical Glass Industrial Standard)-1975, it shows a substantial weight change after the test (weight reduction: at least 0.04 wt. %) and thus has poor water-durability. This means that when moisture condensation takes place on the glass surface, the glass surface is likely to undergo a modification.